The present invention relates, in general to reference circuits, and more particularly, to a semiconductor current source circuit which minimizes output errors for a wide range of supply voltage variations.
Current source circuits which utilize a difference in base to emitter voltages between transistors as a means to develop a constant reference current were well known to the semiconductor industry. Methods used to implement the previous current source circuits resulted in circuits with several transistors stacked between the circuit's supply voltages. The voltage drop associated with the stacked transistors added together and limited the range of voltages over which the supply voltage could vary and yet have the current source continue to produce a desired current. Normally, those stacked transistors also had a body effect voltage which increased the transistor's gate to source voltage and further reduced the circuit's operating range. Generally, the electrical characteristics of those transistors were matched by using long channel transistors because the long channels reduced the effect of process variations on the channel length. However, long channels also increased the transistor's gate to source voltage thereby reducing the current source circuit's operating range even further. Accordingly, it would be desirable to have a current source circuit with fewer stacked devices and with devices having lower voltage drops thereby increasing the current source circuit's useful operating range by operating at lower supply voltages and minimizing the resulting output errors.